Uncertainty quantification of aerothermal coupled flow-material simulations of low-density ablative thermal protection systems
For space missions involving atmospheric entry, the thermal protection system (TPS) is essential to shield the vehicle from the severe aerothermal loads. Whereas TPS materials were fully dense in the past, new lightweight porous fiber/resin composite materials are used for reentry systems nowadays. The optimal design of TPS using these new materials requires however (i) the development of high fidelity numerical models, (ii) the development and calibration of physico-chemical models based on new ablation experiments on porous materials, and (iii) the analysis of the impact of uncertainties stemming from physico-chemical models.
The overarching objective of this project is to contribute to the development of an uncertainty-quantified numerical modeling of the ablation of new porous composite materials and to the analysis of the impact of uncertainty on TPS design. This project has three main axes. (i) First, we will develop a new methodology for representing the process of ablation from resin pyrolysis to char ablation in a unified flow-material approach where the Volume-Averaged Navier-Stokes equations are solved. We will implement this model in a new module of the high-fidelity numerical code ARGO, under development at CENAERO, VKI, and UCL. (ii) Next, we will address the inversion and the uncertainty characterization of physico-chemical models for resin pyrolysis and fiber ablation on the basis of recent experiments. (iii) Finally, we will analyze the impact of the uncertainty in the physico-chemical models on the numerical modeling of ablation of TPS by using nonintrusive stochastic methods. We will compare numerical results with experimental results conducted in the VKI plasma wind tunnel for validation, and we will eventually apply the methodology on engineering problems relevant to in-flight performance prediction and TPS design.
Absolute instability in Plasma jet
During an atmospheric entry, spacecrafts are protected from the extreme temperature developing behind the bow shock by a Thermal Protection System made of ablative materials (TPS). To design the TPS, materials are tested in the VKI Plasmatron Inductively Coupled Plasma wind tunnel, however, relevant phenomena such as ablation, catalysis and transition, are strongly coupled with the quality of the plasma jet flow. Unfortunately, perturbations due to jet hydrodynamic instabilities have been observed experimentally and perturb the experiments. Instabilities found in jets can display two distinct natures: either convective if the flow acts as an amplifier with initial perturbations, either absolute if it acts as an oscillator. However, distinguishing these two types experimentally is not straightforward. Numerically, both convective and absolute types can be accurately estimated by the Linear Stability Theory. Retrieving the frequencies and amplification rates of disturbances growing in the jet. Convective instabilities in the Plasmatron have been previously studied at VKI, without matching all experimental results. Literature studies on absolute instabilities do not cover the operative conditions reached in the facility, leaving the nature of the instabilities observed not completely understood. This research project will investigate the absolute instabilities which may develop in Plasmatron, as a new approach to reduce the uncertainties on the ow field quantities. Modules will be developed and added to the VESTA toolkit, to determine the nature of instabilities developing for all operative conditions. The stability computations will use velocity and temperature proles for a mixture of gases in Local Thermodynamic Equilibrium, fitted to simulations reproducing the experimental conditions. The results will be validated against experimental observations, and will serve to complete absolute instability models and design more efficient TPS.
Analytical and experimental investigation of icing phenomena on wind turbine blades
The biggest technical challenge of the cold climate wind energy is the operation of wind turbines in icing conditions. The super cooled water droplets present in the atmosphere freeze at the impact with the turbine blades. The ice accretion lowers the energy production, degrades the aerodynamic properties and generates unbalanced forces acting on the rotor. Moreover, important safety hazards are caused by the ice shedding phenomenon. In order to limit these negative effects, ice protection systems are used. These systems imply high additional costs and have several drawbacks.
This work proposes an experimental and analytical study of the most encountered icing phenomena in order to investigate the limitations of the widely used electro-thermal de-icing systems. The experiments will be conducted in the VKI icing wind tunnel and will start with the investigation of ice accretion effects over a wind turbine blade. After, applying resistive heating elements on a simplified configuration and using laser illuminated flow visualizations and infrared thermal imaging, the liquid film formed due to the applied heat transfer and the run-back ice phenomenon will be studied. Using also force measurements in the same configuration, the critical parameters for the ice shedding will be determined. An analytical model will be proposed and used to design an optimized de-icing system which will be adapted and tested in a real wind turbine blade configuration. The final goal of this work is to generate an easy to use analytical tool to improve the design and the efficiency of the electro-thermal de-icing systems.
Keywords: Icing, ice shedding, run-back ice, wind turbine, electro-thermal, de-icing, heat transfer, thin films, icing wind tunnel, analytical modeling.
Characterisation of Cryogenic Slush for Future Launcher Engines
Liquid hydrogen is widely used as fuel in rocket engines due to the high specific impulse provided. However, several problems are related to this choice, such us low density, temperature stratification and short holding time. Moreover, strong sloshing can occur due to the liquid state. A promising way to reduce these problems is to use slush, that is a solid-liquid mixture with 15% higher density and 18% higher specific enthalpy. The employment of this form of propellant results in promising new concept rocket engines. However, the presence of solid particle in the liquid suspension changes the flow properties: the rheological behaviour becomes non-Newtonian for high particle concentration and, due to the heat transfer, the solid fraction may partially melt changing slush features. Hence, the standard correlations for the liquid flow in tubes are no longer valid. An observed phenomenon of drag reduction appears to be interesting for improving system efficiency, but it has never been completely understood. Therefore, the optimization of the slush transport through circular pipelines and narrowing, requires the measurement of the slush physical features, along with their modelling.
The goal of this research project is to provide a complete understanding of the slush behaviour at different flow conditions such as velocity, mass fraction concentration and particle distribution. Due to the high risks and extremely high costs involved in using hydrogen, the research will be carried out for slush nitrogen. An intense experimental campaign will furnish a complete database concerning pressure drops, heat transfer and phase change phenomena. This database will be used to design improved engineering correlations as well as revised models for Eulerian-Eulerian numerical approaches available in existing softwares, to be used in the design of slush transport and storage systems.
F. Torres Herrador
Experimental characterization and numerical modeling of space debris degradation during atmospheric reentry
Space debris is predicted to increase drastically in the following decades. This increase of debris can trigger the so-called Kessler syndrome: a cascade effect in which the debris impacts with satellites producing more debris, to ultimately destroy many useful satellites and hinder access to space. It is important to tackle this issue before it becomes a real threat. A cost-effective way to reduce space debris is to force its re-entry on
Earth and full demise. During a re-entry, the high velocities of an in-orbit object will be transformed into heat by friction. The high gas temperatures (about 10 000 K) will lead to the decomposition of the object. However, a complete decomposition may not be guaranteed. For example, a pressure vessel from a Vega rocket of ESA of about 1 m in diameter was found in India in 2016. This debris can be an important threat if it was to fall in an inhabited area. The purpose of this project is to develop new models capable of predicting the demise of this particular type of space debris. The project is divided in four parts:
- Theoretical study and decomposition modeling for the two materials used in pressure vessels: metals (aluminum or titanium) and Carbon Fiber Reinforced Polymer (CFRP).
- Experimental characterization in detailed thermal analysis at small scale (TGA/DSC) and material response characterization in plasma flows (Plasmatron).
- Detailed simulation of space debris demise using a thermal degradation code, PATO, developed in conjunction by NASA Ames and VKI.
- Uncertainty quantification and development of simplified models.
Massively parallel and robust high-order methods for transitional hypersonic flow modeling on unstructured grids: Application to reusable launcher stages
During their ascent and descent trajectories, reusable space vehicles travel primarily in the hypersonic regime (5 < Mach < 25) which is characterized by high speeds, shock waves, chemical dissociation, radiation, viscous interaction… Additionally, the flow experiences along the trajectory various changes in regimes i.e. laminar, transitional and turbulent. Prediction of the onset and extent of the transition from laminar to turbulent regime is particularly challenging and a main subject of ongoing research activities.
The present project consists in developing a fast parallel flow solver based on variable high-order methods, allowing the order of accuracy to be adapted to the local flow field features. Such methods reduce the computational time considerably and allow for calculating more accurate solutions on relatively coarser computational grids than traditional low-order methods.
Furthermore, a novel model to predict laminar-turbulent transition that is independent from the turbulence model will be incorporated.
This solver will be applied to the hypersonic flow around representative reusable launcher stages, e.g. Hexafly-INT and IXV vehicles of the European Space Agency. The results will be compared to experimental and flight data.
The final result will provide the first open-source variable high-order solver able to simulate transitional hypersonic flows and will significantly improve the current state-of-the-art of CFD methods for these challenging applications.